Precise determination of relative and absolute ββ-decay rates ofTe128andTe130

Double beta decay of ${}_{}{}^{128}{}_{}{}^{}\mathrm{Te}$ has been confirmed and the ratio of half-lives for ββ decay of ${}_{}{}^{130}{}_{}{}^{}\mathrm{Te}$ and ${}_{}{}^{128}{}_{}{}^{}\mathrm{Te}$ has been precisely determined as ${\mathit{T}}_{1/2}^{130}$/${\mathit{T}}_{1/2}^{128}$=(3.52±0.11)×${10}^{\mathrm{-}4}$ by ion-counting mass spectrometry of Xe in ancient Te ores, using techniques that reduce interferences due to trapped Xe. We have also detected excesses of ${}_{}{}^{126}{}_{}{}^{}\mathrm{Xe}$ originating in high energy reactions of cosmic ray muons and their secondaries on Te; such reactions make minor contributions to the measured ${}_{}{}^{128}{}_{}{}^{}\mathrm{Xe}$ excesses in the Te ores. The Xe measurements, combined with common Pb dating of the ores, yield a ${}_{}{}^{130}{}_{}{}^{}\mathrm{Te}$ half-life of (2.7±0.1)×${10}^{21}$ yr and thus a ${}_{}{}^{128}{}_{}{}^{}\mathrm{Te}$ half-life of (7.7±0.4)×${10}^{24}$ yr, the longest radioactive decay lifetime measured to date. These results give limits on the effective Majorana mass of the neutrino (<1.1–1.5 eV) and right-handed currents (‖〈η〉‖<5.3×${10}^{\mathrm{-}8}$) comparable to the best obtained from direct neutrinoless ββ-decay searches. They also imply new limits on unconventional Majorons not constrained by measurements of the ${\mathit{Z}}^0$ decay width.